Emulsion polymerization is a special polymerization process, in which olefinically unsaturated monomers poorly soluble in water are emulsified in water with the aid of emulsifiers and are polymerized using water-soluble initiators, such as for example potassium peroxodisulfate or redox initiators. Anionic and/or nonionic surfactants are the key constituents. They guarantee the process of emulsion polymerization through the formation of micelles in the aqueous solution.
The main uses of latices in the coating industry are to be found in the following fields of application: paints and lacquers for decorative purposes and industrial applications, textile treatments, paper coatings and adhesives, i.e. in areas where films are formed from dispersions in the broadest sense after drying.
In most cases, the surfactants or emulsifiers used are physically bound to the surface of the latices and are in a dynamic equilibrium with the water phase. Except for a small part attributable to transfer reactions during the polymerization process, the surfactants are not covalently bonded to the polymer particles. The quantitative distribution of the emulsifier based on sulfosuccinates can be determined, for example, by the radio tracer technique using 35S. As known to the expert, ca. 15% of the emulsifier are fixed to the latices, cf. R. Otto “Radiotracermethode zur Bestimmung der Verteilung schwefelhaltiger Dispergiermittel in Emulsionspolymerisaten”, Isotopenpraxis 20 (1984), 378-381.
For some time, improvements in film formation or improved film stability, for example towards shear forces, or increased water stability have also been attributed to the so-called copolymerizable surfactants, i.e. surfactants which carry a polymerizable group. Accordingly, they may also react as special monomers and participate in the polymerization process.
These special monomers are also referred to in the literature as “surfmers” or non-migratory surfactants. These surfactants are normally incorporated in the polymer particles during the emulsion polymerization process. Because they are chemically fixed to polymer particles, these surfmers—in contrast to the surfactants normally used—cannot readily be desorbed and remain in or on the surface of the polymer particles. In the general opinion of experts, the consequences are advantages in regard to latex stability and water absorption for the latices or films formed from them, cf. Jo Grade “Non-Migratory Surfactants” in European Coatings Journal, 2001, 5, 66-70.
A detailed summary of polymerizable surfactants (surfmers) for emulsion polymerization can be found in: “Surfactants in Polymers, Coatings, Inks and Adhesives”; edited by David R. Karsa; ISBN 0-8493-2808-X; Blackwell Publishing, CRC Press 2003; Chapter 3, pages 71-92. Surfmers can be synthesized, for example, from maleic anhydride. The C═C double bond remaining in the compounds obtained is then the reactive copolymerizable part of the surfactants, cf. pages 72-75 of the publication just cited.
Copolymerizable surfactants of another type are described by S. Poulton in the journal PPCJ, 192 (2002) 4457, pages 24-26. These are alkoxylated (meth)acrylic acid derivatives, so-called bisomers. These bisomers are suitable, for example, for lowering the glass transition temperature of polymers and for the production of polymer dispersions low in VOC.
The laurylic acid semiester of maleic anhydride is known from the literature and commercially available under the name of “Setalux 6774 EPL” from Akzo Nobel Resins, cf. (D. Mestach: “New high performance materials for waterborne acyclic surface”, Double Liaison—Physique, Chemie.& Economie des Peintures & Adhesifs, 2001, No. 523, pp. 33-38). Unfortunately, the poor water solubility of the product is an obstacle to certain applications in the field of emulsion polymerization. Applicants' own “TREM® LF 40”, a sulfosuccinate-based product, is also known. Applications are described inter alia in: M. B. Urquiola, V. L. Dimonie, E. D. Sudol and M. S. El-Aasser, “Emulsion Polymerization of Vinyl Acetate Using a Polymerisable Surfactant. II. Polymerization Mechanism”, J. Polym. Sci. Part A: Polym. Chem., 30 (1992), 2631.
It is also known that the solubility of maleic acid semiesters in water can be improved by reacting them with ethylene oxide, the ethylene oxide reacting at the carboxyl group of the maleic acid semiester, cf. the above-cited publication “Surfactants in Polymers, Coatings, Inks and Adhesives”, pp. 74-75.
DE-A-19831706 describes a process for the production of polymers by emulsion polymerization of unsaturated monomers, in which the copolymerizable emulsifiers used are maleic acid esters corresponding to formula (I):R1OOC—CH═CH—COO—(BO)z(PO)y(EO)xR2  (I)in which R1 is a linear or branched alkyl and/or alkenyl group containing 6 to 24 carbon atoms, R2 is an alkyl group containing 1 to 4 carbon atoms, BO is a butylene oxide unit, PO is a propylene oxide unit and EO is an ethylene oxide unit and the indices x, y and z independently of one another stand for 0 or for numbers of 1 to 100, with the proviso that at least one of the indices x, y and z is not 0.